For several decades now, integrated circuit and laser technologies have doubled in performance approximately every 18 months. These technologies have been used to support a rapidly-growing demand for global communications capacity. This demand is currently growing much faster than the underlying rate of improvement of the supporting technologies. As an example, communication traffic through the Internet has recently been doubling every nine months. The demand for additional current bandwidth is severely stressing the capabilities of current electronic and optical technologies.
In particular, the Ethernet local area network standard has progressively increased in speed by factors of ten, starting at 10 megabit per second (Mb/s) in 1982. Proposals for a 10 gigabit/second (Gb/s) Ethernet standard were made in 1999. A prior Ethernet standard used an 8b/10b line code described by A. X. Widmer and P. A. Franaszek in A DC-Balanced, Partitioned-Block, 8b/10b Transmission Code, 27 IBM J. RES. AND DEV., (1983 September) for transmitting serial data at 1 Gb/s. In the 8b/10b line code, each 8-bit input word is represented by a 10-bit code that is transmitted on the data link. In exchange for this 25% overhead, 8b/10b coding provides DC balance, and a guaranteed transition density. The 10-bit code additionally has the ability to represent an assortment of control words used for signaling and framing.
Re-using 8b/10b coding for sending information at 10 Gb/s was considered in the proposed 10 Gb/s Ethernet standard. However, using this technique would result in having to transmit at a baud rate of 12.5 Gbaud, i.e., 12.5 Gb/s.
With currently-available laser fabrication technology, manufacturing a laser capable of modulation at 12.5 Gb/s at a modest price is considered to be quite difficult. However, laser systems currently exist for use in systems conforming to the OC-192 SONET telecommunications standard. Such systems operate at signaling rates of 9.95328 Gb/s. However, these commercially-available lasers do not have enough performance margin to run at more than 25% faster than their design speed.
One way to enable the lasers designed for use in SONET telecommunications systems to be used in the proposed 10 Gb/s Ethernet standard would be to design a simple and robust coding scheme with a lower overhead than 8b/10b line code. In principle, this goal can be achieved using a block code in which words of M bits are represented by an N-bit code and in which the ratio of N:M is less than 10:8.
A potential coding scheme having a lower overhead than 8b/10b line code is that used in the SONET telecommunications standard. The SONET coding scheme assures DC balance by using a scrambling system, and has an overhead of about 3%. However, the scrambling system used in the SONET coding scheme uses two layers of polynomial scrambling to achieve an adequate level of protection. This two-layer scheme is complex to implement. Moreover, the SONET coding scheme has a complex framing protocol that is difficult to implement at low cost. The SONET coding scheme would also have to be modified to add an extra level of encoding to support Ethernet packet delimiting. Such an extra level of coding would probably increase the overhead of the SONET coding to 7% or more. In addition, it is thought that the networking community would find the wholesale adoption of a telecommunications standard to be unpalatable. The performance and political difficulties just described would make it difficult for a standard based on the SONET coding scheme to be adopted as a new Ethernet standard.
Another potential coding scheme having a lower overhead than 8b/10b line code is that known as CIMT. This coding scheme is described in U.S. Pat. No. 5,022,051 of Crandall et al. and U.S. Pat. No. 5,438,621 of Hornak et al. The CIMT code is an (M)b/(M+4)b code that can be configured to have a lower overhead than 8b/10b line code by making the value of M sufficiently large. However, for large values of M, the CIMT code is difficult to implement due to the need to compute the DC balance of an incoming block of M bits, and the need to compute a running DC balance of the transmitted bits in real time.
U.S. Pat. Nos. 6,718,491 and 6,650,638, which are assigned to the same assignee as the present disclosure, disclose a 64b/66b coding scheme that addresses the problems identified above. In the disclosed coding scheme, a 2-bit “master transition” is appended to each block of data to form a frame. When either all control information, or all data, is being transmitted, the master transition is a constant value, and the resulting periodic signal causes undesirable spectral peaks in the frequency domain.